Heat Affected Zone Softening Research Articles

Effect of V and Mn addition on the HAZ softening and tensile properties of friction stir welded martensitic steel

This study explores the suppression of heat-affected zone (HAZ) softening in friction stir welding (FSW) joints formed above the A3 temperature of martensitic steels by leveraging short-time secondary hardening induced by vanadium (V) addition. The HAZ microstructure was examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atom probe tomography (APT). Mechanical properties were evaluated through Vickers hardness (HV) measurements and tensile tests. Analysis of hardness distribution indicates effective HAZ softening suppression with V addition, attributed to the precipitation of fine and high-density V carbides within the HAZ. The inhibition of HAZ softening was correlated not only with V content but also with the A1 temperature of the steels, regulated by the manganese (Mn) content. However, at 1 mass% V content, intergranular brittle fracture occurred within the HAZ due to continuously distributed fine carbides and Mn segregation along the prior austenite boundary.

Softening mechanism and failure behavior of 9Cr oxide dispersion strengthened steel resistance spot welded joint

Oxide dispersion strengthened (ODS) steel is one of the most promising materials for nuclear applications. However, there are fewer studies on the spot welding of ODS steel, and the softening mechanism of ODS steel resistance spot welded joint is unclear. The microstructural features of the ODS resistance spot welded joint was characterized to analyze the softening mechanism. The failure behavior and tensile shear performance were studied combined with finite element simulation. The results showed that the fusion boundary has the lowest hardness and the softening zone is divided into fusion zone(FZ) softening and heat affected zone(HAZ) softening according to different location in the joint. In FZ softening, the softening is mainly caused by the presence of δ-ferrite and fewer oxides as well as lower dislocation density. In HAZ softening, the softening is mainly caused by larger grains, less oxides. There are two types of failure, the fracture initiated at HAZ softening shows a brittle feature, while initiated at FZ softening shows a ductile feature. The softening leads to a reduction in tensile-shear strength. Thus, the control of softening zone is critical to improve the reliability of welded joints.

Effect of Interface Form on Creep Failure and Life of Dissimilar Metal Welds Involving Nickel-Based Weld Metal and Ferritic Base Metal

For dissimilar metal welds (DMWs) involving nickel-based weld metal (WM) and ferritic heat resistant steel base metal (BM) in power plants, there must be an interface between WM and BM, and this interface suffers mechanical and microstructure mismatches and is often the rupture location of premature failure. In this study, a new form of WM/BM interface form, namely double Y-type interface was designed for the DMWs. Creep behaviors and life of DMWs containing double Y-type interface and conventional I-type interface were compared by finite element analysis and creep tests, and creep failure mechanisms were investigated by stress-strain analysis and microstructure characterization. By applying double Y-type interface instead of conventional I-type interface, failure location of DMW could be shifted from the WM/ferritic heat-affected zone (HAZ) interface into the ferritic HAZ or even the ferritic BM, and the failure mode change improved the creep life of DMW. The interface premature failure of I-type interface DMW was related to the coupling effect of microstructure degradation, stress and strain concentrations, and oxide notch on the WM/HAZ interface. The creep failure of double Y-type interface DMW was the result of Type IV fracture due to the creep voids and micro-cracks on fine-grain boundaries in HAZ, which was a result of the matrix softening of HAZ and lack of precipitate pinning at fine-grain boundaries. The double Y-type interface form separated the stress and strain concentrations in DMW from the WM/HAZ interface, preventing the trigger effect of oxide notch on interface failure and inhibiting the interfacial microstructure cracking. It is a novel scheme to prolong creep life and enhance reliability of DMW, by means of optimizing the interface form, decoupling the damage factors from WM/HAZ interface, and then changing the failure mechanism and shifting the failure location.

Dissimilar joining of duplex AISI 2304 stainless steel/austenitic AISI 321 stainless steel using resistance spot welding technique: Microstructural evolutions, tensile-shear properties, and fracture analysis

In this work, microstructural characteristics and mechanical performance of spot weld joints of AISI 321 austenitic stainless steel (ASS)/AISI 2304 duplex stainless steel (DSS) were experimentally assessed. The welding current and time were varied to determine their effects on the failure load/energy and mechanism of dissimilar welds. The weld metal (WM)’s microstructure had a ferritic matrix as well as a distribution of grain boundary, Widmanstätten, and transgranular austenite. Some precipitates were found in the weld. The heat-affected zone (HAZ) of the AISI 321 ASS did not experience extensive phase evolutions and only grain coarsening occurred. This led to the HAZ softening. While extensive phase transformations were identified in the HAZ of the AISI 2304 DSS. Its microstructure contained a ferrite matrix with Widmanstätten and grain boundary-type austenite. Such microstructure resulted in the HAZ strengthening. Alteration of the welding condition from 1 kA – 2 s to 3 kA – 2 s raised peak load (tensile-shear strength) from 9.7± 0.8 to 16± 1 kN and energy absorbed to fracture from 15.52 ±1 to 51.2± 2 J. Enhanced WM size was responsible for them. Finally, as welding condition reached 4 kA – 2 s, they would decrease to 14.4±1 kN and 41.76± 3 J, respectively. It arises from surface splash. Once the welding condition varied from 3 kA – 1 s to 3 kA – 3 s, the weld performance was promoted i.e., improvement of peak load from 13± 1 to 19.6± 0.5 kN and energy absorbed to fracture from 23.4±2 to 49±3 J. It results from the WM enlarged. Ultimately, welding condition of 3 kA – 4 s causes a degradation in the weld performance i.e., drop of peak load to 15.8±1 kN and energy absorbed to fracture to 33.18± 2 J. This was on account of surface splash. With reference to the fracture analysis, interfacial, single pull-out, and double pull-out fracture modes were characterized.

Numerical and experimental study of underwater friction stir welding of 1Cr11Ni2W2MoV heat-resistant stainless steel

Due to the benefits of solid-state joining, friction stir welding (FSW) has seen an increase in applications in the aircraft and automotive industries. However, short tool life and heat-affected zone (HAZ) softening are the two main issues with the FSW of advanced high-strength steel. In this study, the underwater friction stir welding (UFSW) of 1Cr11Ni2W2MoV steel was investigated and compared to the Normal friction stir welding (NFSW). Besides, a 3-D thermo-mechanical finite element model was developed to understand the effect of the different cooling mediums and tool rotation rates on the thermal cycles, plastic deformation, tool wear, and microstructure evolution. The simulation results revealed that the stir zone (SZ) of the UFSW exhibits lower peak temperatures and smaller plastic strain compared to the NFSW. The experimental results showed that a high tool rotation rate can be used during UFSW without causing overheating of the workpiece and tool materials resulting in high joint quality and low tool wear. The M23C6 and Fe3C carbide fractions in the HAZ at UFSW are lower than that at NFSW. The UFSW reduces the soft HAZ's width and increases its hardness. The ultimate tensile strength was increased by 17 %, and the HAZ width was decreased by 35 % at UFSW compared to that at NFSW.

Unveiling heat-affected zone softening in dissimilar AZ31B and WE43 alloy welds

In this study, AZ31B-H24 and WE43-T5 Mg alloys were joined using friction stir welding. The effect of base material position, tool offset and rotational speed on the microstructure and mechanical properties was investigated. With AZ31B on the advancing side (AS), distinct layers of both materials were observed in the nugget zone (NZ), while material mixing in the NZ was better with WE43 on the AS. A maximum joint strength of 207 MPa (97% of AZ31B) was observed with AZ31B on the AS without tool offset and a tool rotation speed of 600 RPM. The failure location was at the thermo-mechanically affected zone/heat-affected zone of AZ31B side when AZ31B was on the AS, while joints with WE43 on the AS failed in the NZ.

Fracture, deformation route, and mechanical performance of welded cold-formed ultra-high strength steel S1100

This study investigates the influence of cold-forming on the weldability and fracture behavior of ultra-high strength steel S1100. The mechanical performance and fracture of such steels while simultaneously subjected to cold-forming and welding still require further research, although cold-forming and welding have crucial roles in fabricating economical structures with relatively acceptable strength-to-weight ratios. Hence, S1100, as a thermomechanically processed ultra-high strength carbon steel, was subjected to bending with different radii (representing various levels of cold-forming) and welded with gas-metal arc welding. Next, the strength, ductility, and toughness of the welded samples were evaluated using tensile tests. In addition, digital image correlation was used with the tensile tests to study the plasticity and final failure. Further, macrographs by optical microscopy and fractography by scanning electron microscopy were utilized to investigate the fracture locations and mechanisms of the welded joints. The results confirmed the reliability of the welded joints in steel structures, and the fracture of the welded material was identified as ductile, regardless of its cold-forming degree. Heat-affected zone softening, as a common drawback associated with welded ultra-high strength steel, did not adversely affect the fracture mechanism of welded S1100, even in its cold-formed state.

Enhancing tensile properties of pulsed CMT–MIG welded high strength AA2014-T6 alloy joints: Effect of post weld heat treatment

The main objective of this investigation is to study the effect of post weld heat treatments (PWHTs) on tensile properties, hardness, and microstructure of pulsed CMT–MIG (cold metal transfer arc–metal inert gas) welded AA20214-T6 aluminum alloy joints. The welded joints were subjected to PWHT of artificial aging (AA), solution annealing treatment (ST) and ST + aging (STA). The tensile properties and microhardness of joints were evaluated. The microstructure of joints was studied using optical microscope (OM) and transmission electron microscope (TEM). The fractured surface of tensile specimens was analyzed using scanning electron microscope (SEM). Results showed that the tensile properties and hardness of as welded and PWHT joints are inferior compared to base metal (BM). This mainly refers to the microstructural heterogeneity in different regions of joints and softening of heat affected zone (HAZ) induced by the weld thermal cycle. The PWHTs of AA and ST did not show significant effect on tensile strength and hardness of AW joints. However, the slight reduction in elongation was observed in AA and ST joints. The STA joints showed higher joint efficiency of 71.6%, than other joints by compromising on the elongation. This refers to the greater precipitation of hardening precipitates in STA joints compared to AA and ST joints. It exhibited the higher tensile and yield strength of 326 MPa and 266 MPa and the lowest elongation of 3.8%. The STA joints showed 28.35%, 38.28% and 57.77% reduction in tensile strength, yield strength and elongation compared to BM respectively. All the tensile specimens of joints failed in HAZ owing to the lower hardness. This refers to dissolution of precipitates in HAZ. However, the HAZ softening is less severe in STA joints than AW, AA, and ST joints.

Strength calculation and microstructure characterization of HAZ softening area in 6082-T6 aluminum alloy CMT welded joints

Degradation of mechanical properties in the heat-affected zone (HAZ) is a common issue when welding 6082-T6 aluminum alloy. Even for cold metal transfer (CMT) welded joints with little heat input, it is always challenging to avoid this problem. In this study, we examined the effects of various heat inputs on the softening of CMT welded 6082-T6 aluminum alloy joints as well as the mechanism underlying the softening of HAZ. The hardness of the welded joint at the HAZ softening area directly decreases as the heat input increases. The softening of HAZ is the most severe under the impact of thermal cycling, with a softening zone width of up to 5.5 mm and a minimum hardness of about 61 HV, reaching 58.65% of the base material (BM). According to the strength model established in the study, the most significant factors affecting the strength of welded joints were the weakening of precipitation strengthening and solid solution strengthening due to welding thermal cycling. These factors accounted for 51.51% and 30.26% of the total strength, respectively. Compared with BM, the contribution of precipitation strengthening and solid solution strengthening in HAZ softening area decreased by 85 MPa and 23 MPa, respectively. Where the weakening of precipitation strengthening is mainly due to the β" and β' phase transitions induced by welding thermal cycling. The decrease in solid solution strengthening is then mainly influenced by the change in the content of Mg and Si elements in the matrix. The revelation of softening mechanism provides theoretical support for how to control the loss of mechanical properties of welded joints of Al-Mg-Si aluminum alloy.

Mechanism of suppressing HAZ softening in friction stir welded martensitic steel through hardening phenomenon initiated by adding vanadium

In this study, heat-affected zone (HAZ) softening occurred in friction stir welding (FSW) joints of martensitic steels is suppressed through the short-time secondary hardening phenomenon initiated by adding V. Adding V significantly suppresses the HAZ softening owing to the precipitation of the extremely fine and high-density V carbides in HAZ. Vickers hardness (HV) profiles of the FSW joints were measured to evaluate the degree of HAZ softening. The degree of inhibition in HAZ softening was found to be related to the amount of V present and the Ac1 temperature of the steels, which can be controlled by the quantity of Mn. These relationships were elucidated quantitatively via transmission electron microscope (TEM) and 3D atom probe-tomography (3DAPT) together with hardening theory after investigating multiple areas of the V carbides in FSW joints containing different V and Mn contents.

Resistance spot weldability of Fe66Cr16.5Ni14.1Si3.4 advanced high strength steel using D-optimal design of experiment method

Using advanced high-strength steels in automobiles has created a new challenge to achieve acceptable welds. The present study evaluates the weldability of the new generation of advanced high-strength steels in the FeCrNiSi alloy group. Hence, utilizing the design of experiment (DOE), it was obtained how the parameters namely the welding current, welding time, and electrode force affect the final properties. Additionally, the acceptable range of parameters for achieving an acceptable weld nugget in terms of size and strength was obtained. Accordingly, the nugget size effects on mechanical properties, namely energy absorption capability, failure mode in tensile-shear test, and microhardness were evaluated. The parameters were statistically significant and showed a high degree of reliability (95%) in terms of nugget size. The range of acceptable parameters for 1 mm thick steel and type A electrode was 9–21 cycles for welding time, whereas for welding current were 4–7 KA, 5 to 6.5 KA, and 5.7 to 6.5 KA at 2, 3, and 4 KN electrode force, respectively. Due to the grain growth in the heat-affected zone (HAZ), the HAZ softening was observed in the microhardness profile. All samples demonstrated desired fracture mode due to the HAZ softening, the ductile austenitic microstructure, the absence of brittle phases, and easy nugget rotation in the tensile-shear test. By increasing the nugget diameter from 3 to 5.1 mm, the peak load and the energy absorption of the spot welds exceeded from 5.4 KN to 6.5 KN and from 10 to 31 J, respectively.

Strain-based fracture analysis for internal surface cracks of X80 pipe girth welds

The girth weld is the weakest region of the entire pipeline. When the pipeline crosses areas with complex terrain or where natural disasters are frequent, the pipeline may be subjected to large displacement effects and large plastic deformations. In these scenarios, the traditional stress-based assessment methods will no longer be applicable. Therefore, it is of great importance to develop strain-based assessment methods for pipelines. This paper investigates the effects of crack size, strength mismatch, heat affected zone (HAZ) width and HAZ softening/hardening coefficient on the crack driving force (CDF) of weld center crack (WCC) and HAZ center crack (HAZCC) on the inner surface of X80 pipes using finite element method. The results show that the CDF is significantly affected by crack size, crack location, strength mismatch, HAZ softening coefficient but slightly affected by HAZ width when HAZ width is greater than 2 mm. The CDF of WCC is greater than that of HAZCC under large strain conditions with HAZ width of 2 mm. The distribution of the J-integral at the crack tip is then investigated and the results show that the maximum value of the J-integral does not necessarily occur at the deepest point of the crack tip. Finally, the factors influencing the failure assessment curve (FAC) of WCC are investigated and the lower bound strain-based failure assessment diagram (SB-FAD) considering strength mismatch and HAZ softening coefficient is developed with varying crack depths. This study can provide a reference for the safety assessment of crack-like defects on the inner surface of X80 pipeline girth welds.

Tensile shear fracture load bearing capability, softening of HAZ and microstructural characteristics of resistance spot welded DP-1000 steel joints

Abstract The main objective of this investigation is to enhance the tensile shear fracture load (TSFL) bearing capability and minimize softening in heat affected zone (HAZ) of resistance spot welded DP-1000 steel spot joints for automotive applications. The lap tensile and cross tensile shear fracture load tests (LTSFL and CTSFL) were conducted. The process parameters were optimized using numerical and graphical optimization techniques to maximize the TSFL capability of spot joints. The microstructure of spot joints was studied using optical microscopy and correlated to TSFL and hardness of spot joints. Results showed that DP-1000 steel spot joints made using the welding power of 70 W, welding time of 1.0 s, electrode pressure of 4.25 MPa showed maximum LTSFL of 22 kN and CTSFL of 9.1 kN. The parametric optimization showed 46.66, 45.77, 22.33 and 9.87% increase in LTSFL, CTSFL, nugget zone hardness and HAZ hardness of DP-1000 steel spot joints. The higher TSFL and nugget hardness of spot joints is mainly attributed to the evolution of finer martensitic sandwiching ferrite phases in nugget zone and lower softening of HAZ than other joints. Welding power showed significant influence on TSFL of spot joints followed by welding time and electrode pressure.

Application of a high-speed infrared thermographic camera to the study of HAZ softening in S960 welded joints

Hot strip rolling of low-carbon advanced high-strength (AHS) sheet steels is difficult because their products require specific chemical compositions to achieve excellent mechanical properties. The formation of a heat-affected zone (HAZ) during welding is therefore a challenge in terms of maintaining high strength values. This fact complicates the formation of the softening area when the hardness in the HAZ rapidly decreases. According to the continuous cooling transformation diagram (CCT), the chemical composition of the base metal is directly related to the fine-grained martensitic microstructure arising from very high cooling rates and microalloying up to 0.2 wt% by Nb, V, and Ti. The gas metal arc welding (GMAW) was used to perform the study, what presents a challenge for thin sheets because of very narrow HAZ formation, suscepible to HAZ softening phenomenon. The width of the HAZ was about 6 mm and determined by applying an infrared thermographic camera to measure the real-time temperature development. The significant reduction in microhardness, known as the softening phenomenon, occurred in the intercritical heat-affected zone or at the boundary between intercritical and subcritical HAZ, according to real-time diagnostics of the temperature development and the microhardness results. The amount of heat produced during welding plays an important role in this context and is directly connected to the heat input of the welding process. The softening area can be reduced and its impact on the mechanical properties can be lowered by appropriately modifying the welding conditions, e.g., optimising the heat input. The purpose of this paper is to point out the change in temperature during the welding process.

Shared mechanism between flow drill screw and friction stir welding and its impact on failure prediction of steel-aluminum joints

Severe plastic deformation and heat generation during the flow drill screw (FDS) process led to profound changes in the microstructures and mechanical properties of the heat-affected zone (HAZ). Microhardness measurements, electron backscattered diffraction (EBSD), and forming process simulation results showed that the HAZ was located within 1.0 mm from the thread tip. According to the simulated results of the joint forming process, the HAZ was divided into four subzones. The challenge in the numerical simulation of FDS joints lied in how to accurately acquire the mechanical properties of HAZ with limited size. In the present study, large-sized equivalent specimens were prepared with the friction stir welding (FSW) process. Based on the mechanical testing results of the equivalent specimens, a three-dimensional numerical model was established. Simulation results revealed that considering the material softening of HAZ was crucial for accurately predicting the mechanical behaviour of the joints under the tensile loading condition. The prediction accuracy of the peak force and failure displacement was improved by 7.0% as compared with that obtained using the numerical model without considering the material softening of HAZ. Under shear and mixed loading conditions, the error difference of predicted peak force and failure displacement with the two models was less than 3.0%. The deformation and failure characteristics of the joints could be accurately predicted by both the models.

Investigation of microstructure, and mechanical properties of dissimilar high and ultra-high steel welded joints: application for extreme climate conditions

The paper focuses on the technical challenges of producing high-quality welds in modern extreme climate conditions structures, as welds are typically the weakest part of welded structures. Welding is particularly difficult with high-strength and ultra-high-strength steels (HSS-UHSS), which are used in structures to reduce weight. The microstructural compositions and mechanical properties of dissimilar high-strength and ultra-high-strength steels were investigated in this study, which was performed with three different heat inputs (0.8, 1.2, and 1.8 kJ/mm). There was a 2.3Cr, 0.4Si, and 2.8Mn increase on the FGHAZ microstructure of the S960QC side, confirming the temperature increase in that zone. Microhardness results show softening (160 HV5) in the E500 side's fine grain heat-affected zone (FGHAZ). Bending test results show that when the maximum force applied was 4000N, the fracture angle was close to 149°, and that the fracture zone was oriented exclusively in the FGHAZ, which had the higher softening zone. Tensile results show the fracture zone, which was oriented in the E500 side's FGHAZ. It was suggested that a heat input of 1.2 kJ/mm be applied to the weld dissimilar joint of TMCP E500-S960QC, which will be beneficial for extreme climate conditions.

Tensile Strain Capacity Prediction Model of an X80 Pipeline with Improper Transitioning and Undermatched Girth Weld.

As an important component of strain-based design, the tensile strain capacity (TSC) concept has been extensively used for pipelines that experience expectable plastic strain for both installation and service. However, some stress-based designed pipelines have experienced unforeseen plastic strain in the past decade that resulted in failure. It seems that the tensile strain capacity has gradually become an important requirement for geohazard risk management and pipeline maintenance of stress-based design pipelines. The tensile strain capacity of an X80 pipeline is investigated. The assessment in this work was based on the fracture initiation–control-based limit state. This limit state corresponds to the onset of stable tearing and generally provides a reasonably conservative estimate. Besides that, factors such as wall thickness, material’s strain hardening capacity, toughness, weld strength mismatch, HAZ (heat-affected zone) softening, pipe wall thickness, high–low misalignment, and internal pressure were also investigated to construct a prediction model of the X80 vintage pipeline.

Friction stir welding of severely plastic deformed aluminum using (Al2O3 + graphite) hybrid powders: grain structure stability and mechanical performance

Welding of severely plastic deformed aluminum sheets, with high stored strain energy, faces a serious concern, which is decrease in strength of weld and heat-affected zone (HAZ) due to instability in microstructure and related grain growth. In this work, two-passes constrained groove pressed (CGPed) Al-1050 sheets were welded via friction stir welding (FSW), without and with addition of a hybrid powder including 50 vol.% α-Al2O3 nanoparticles+50 vol.% graphite micrometric powders (∼1 vol.% of hybrid powders in stir zone). Al2O3 nanoparticles may result in grain refinement in stir zone by grain boundary pinning. Lubricating nature of graphite may reduce welding heat input resulting in lower degree of HAZ softening. Addition of hybrid powders improved thermal stability of grain structure and increased hardness of stir zone and strength of weld after three passes of FSW and at high rotation speed (ω) to traverse speed (v) ratio. With one-pass FSW, a sound weld was not achieved. However, higher weld efficiency (with transition of failure location from stir zone to HAZ) was obtained when three-pass FSW was carried out due to better material flow, superior distribution of powders, and elimination of cavities. Due to presence of graphite, effect of increase in pass number was negligible on HAZ softening. Finer distribution of hybrid powder was achieved at high ω/v ratios. Weld efficiency was improved from ∼50% (strength of 62 MPa) in one-pass FSW at ω/v = 25 r/mm to a maximum of ∼80% (strength of 101 MPa) after conducting three-pass FSW at ω/v = 70 r/mm.

Improving heat-affected zone softening of aluminum alloys by in-situ cooling and post-weld rolling

The welding of aluminum alloys (Al-alloys) is of great practical relevance in several industries, but the process can be problematic due to the prevalence of a well-known softening phenomenon in the fusion zone (FZ) and heat-affected zone (HAZ) surrounding the weld. This phenomenon has a unique gradient-like characteristic such that the HAZ near the FZ softens the most and the softening degree decreases as the distance from the FZ increases. To minimize the adverse effect of joint softening, this study proposes the use of a novel hybrid manufacturing process for joining AA5083-O and AA6061-T6 dissimilar Al-alloys, using tungsten inert gas (TIG) welding with filler wire under in-situ cooling conditions using specialized welding fixtures followed by the subsequent rolling of the weld reinforcement. The results show that welding under in-situ cooling conditions can improve the joint softening gradient, and the rolling of the weld reinforcement can introduce a strain hardening gradient, that greatly improves the integrity of the joint. By employing the proposed method, the fracture is successfully transferred from the HAZ of the AA6061-T6 side, where it normally occurs, to the base metal (BM) of the AA5083-O side. The proposed process has an extremely high impact potential as it can greatly increase the adoption of Al-alloys in the manufacturing of lighter automotive vehicles which will improve their efficiency and performance.

Effect of Nb Addition and Heat Input on Heat-Affected Zone Softening in High-Strength Low-Alloy Steel.

The effect of both Nb content and heat input on the softening phenomenon of the heat-affected zone (HAZ) of low-alloy high-strength steel was studied through welding thermal simulation experiments. The microstructure evolution, density variation of geometrically necessary dislocation, microhardness distribution and the second phase precipitation behavior in HAZ was characterized and analyzed by combining the optical microscope, scanning electron microscope, high-resolution transmission electron microscope with microhardness tests. The results showed that the softening appeared in the fine-grain HAZ (FGHAZ) of the low-alloy high-strength steel with the polygonal ferrite and bainite microstructure. With an increase in Nb content, the FGHAZ softening was inhibited even with high heat input; however, the hardness shows little variation. On the one hand, the increase in the Nb content increased the volume fraction of high-strength bainite in the FGHAZ. On the other hand, the remarkable strengthening was produced by the equally distributed precipitation nanoparticles. As a result, the two factors were the main reason for the solution of the FGHAZ softening problem in the low-alloyed high-strength steel with the mixed microstructure of ferrite and bainite.